Kyanite formation and element fractionation in the high-al eclogites from the sulu UHP metamorphic terrane

Eclogite compositions are critical parameters for understanding chemical evolution in the Earth, particularly in subduction zones. A group of eclogites from the Sulu ultra-high pressure (UHP) metamorphic terrane in eastern China shows uncommon petrographic and compositional features. They are characterized by (1) zoisite porphyroblasts coexisting with inclusion-free, inclusion-containing, and network kyanite, (2) high Al₂O₃ of 18.4 - 29.2% with high MgO of 8.59 - 11.3%, and (3) intensive HFSE-REE fractionations represented by [Zr/Sm]_ch, and [Ti/Gd]_ch ratios of 0.1 - 3.9 and 1.1 - 9.0, respectively. High-pressure fluids played a major role on developing these features. Kyanite shows two textural varieties. Kyi is inclusion-free and in apparent textural equilibrium with garnet and omphacite, implying formation from plagioclase breakdown. Ky_II includes kyanite networks and porphyroblasts; the former occurs mostly in garnet, whereas the latter encloses garnet and omphacite grains. Ky_II were crystallized at the expense of garnet and omphacite during eclogite-fluid interaction. The low [HREE]_ch ratios of 1 - 2 indicate that the protoliths were arc cumulates. The well-defined inverse Al₂O ₃-SiO₂ and Al₂O₃-CaO correlations are not the characteristics of igneous precursors; therefore, must reflect metamorphic modifications. The comparison to mafic cumulates shows that the low-Al samples are compositionally better representatives of protoliths. Mass balance calculations point to an olivine gabbronoritic protolith. The intensive HFSE-REE fractionations reflect compositional differences between two sample groups. Group I samples have superchondritic HFSE-REE ratios [(Nb/La, Zr/Sm, Ti/Gd)_ch = 2-7] with depletions in LREE. In contrast, group II samples show HFSE depletions and LREE enrichments. These compositional differences were explained as reflecting element mobility during eclogite-fluid interaction. Released at temperatures > 750°C, the high-pressure fluids were enriched in HFSE, REE, and LILE. Upon migration, the fluids first precipitated HFSE into the group I samples, then, LILE and LREE into the group II samples. This model is supported by the occurrence of zoisite porphyroblasts in the group II samples, and interstitial zircon and clusters of small rutile grains along annealed fractures in the group I samples. Our interpretation for the observed compositional variations implies limited element mobility during subduction and exhumation of continental lithosphere, consistent with existing models, which proposed that the chemical flux to metasomatized mantle wedge is mainly from subducted sediments.